RE: [RFC] /dev/ioasid uAPI proposal

["Tian, Kevin" <kevin.tian@intel.com>]

> From: Paolo Bonzini <pbonzini@redhat.com>
> Sent: Saturday, June 5, 2021 2:22 PM
> 
> On 04/06/21 19:22, Jason Gunthorpe wrote:
> >   4) The KVM interface is the very simple enable/disable WBINVD.
> >      Possessing a FD that can do IOMMU_EXECUTE_WBINVD is required
> >      to enable WBINVD at KVM.
> 
> The KVM interface is the same kvm-vfio device that exists already.  The
> userspace API does not need to change at all: adding one VFIO file
> descriptor with WBINVD enabled to the kvm-vfio device lets the VM use
> WBINVD functionality (see kvm_vfio_update_coherency).
> 
> Alternatively you can add a KVM_DEV_IOASID_{ADD,DEL} pair of ioctls.
> But it seems useless complication compared to just using what we have
> now, at least while VMs only use IOASIDs via VFIO.
> 

A new IOASID variation may make more sense in case non-vfio subsystems
want to handle similar coherency problem. Per other discussions looks 
it's still an open whether vDPA wants it or not. and there could be other
passthrough frameworks in the future. Having them all use vfio-naming
sounds not very clean. Anyway the coherency attribute must be configured
on IOASID in the end, then it looks reasonable for KVM to learn the info
from an unified place.

Just FYI we are also planning new IOASID-specific ioctl in KVM for other
usages. Future Intel platforms support a new ENQCMD instruction for
scalable work submission to the device. This instruction includes a 64-
bytes payload plus a PASID retrieved from a CPU MSR register (covered
by xsave). When supporting this instruction in the guest, the value in
the MSR is a guest PASID which must be translated to a host PASID. 
A new VMCS structure (PASID translation table) is introduced for this
purpose. In this /dev/ioasid proposal, we propose VFIO_{UN}MAP_
IOASID for user to update the VMCS structure properly. The user is
expected to provide {ioasid_fd, ioasid, vPASID} to KVM which then
calls ioasid helper function to figure out the corresponding hPASID
to update the specified entry.

Thanks
Kevin